This invention relates, in general, to audio power amplifiers, and more particularly, to the adjustment of the operating point of the output stage.
The present invention has utility in adjusting the bias current in an amplifier circuit, particularly in audio amplifiers where it is desired to operate in Class A operation, while minimizing power dissipation.
It is desirable to adjust the output stage bias current in a Class A amplifier so as to insure proper operation without excessive power dissipation. In high power amplifiers, the quiescent current can be quite high, resulting in very high temperatures.
It is common to use some scheme to reduce the quiescent current when there are no or low input signals, so as to reduce the power dissipation. The magnitude of the input signal is commonly utilized. That is, the bias is made proportional to the input signal level; but these schemes can rarely maintain an adequate adjustment range based solely upon the input signal.
It is desirable therefore, to dynamically adjust the bias current so as to maintain operation in Class A operation while minimizing overall power consumption.
Why adjust the output stage bias current rather than maintain a fixed class A bias level for all load conditions? While class A operation is regarded as the best sounding amplifier mode, it is very inefficient. For a pure class A amplifier design the theoretical efficiency is only 25%; the other 75% is dissipated as heat. By definition, however, the output stage bias current need only be equal to the load current. Therefore, by measuring the load current and adjusting the bias current accordingly, class A bias can be maintained without excessive power dissipation.